1. Field of the Invention
The present invention relates to a method for forming directly on a working model of a tooth an inner crown, primarily made of a noble metal, of a composite-layered crown for restoring crowns, which comprises an inner crown and an outer crown and is designed to be used for the restoration of crown (hereinafter simply referred to as the composite-layered crown(s)), and to an apparatus for carrying out said method.
2. Statement of the Prior Art
Heretofore, the inner crowns of the composite-layered crowns have been made by the use of either a metal casting or a metal foil. According to the method making use of a metal casting, the inner crown of the composite-layered crown is made by the precise lost-wax casting process, and according to the method making use of a metal foil, a metal foil to form the inner crown is covered over a working model of a tooth without recourse to the lost-wax casting process, then pressed against it by means of a spatula for pressing a foil, and finally swaged over that working model by a swager.
When the inner crown of the composite-layered crown is made by the use of the metal casting, a wax pattern is formed of a working model of a tooth on which the state of the tooth to be restored is reproduced (and which may hereinafter be simply called as the working model of a tooth), provided with a sprue wire, and invested in an investment. After the investment has been set, it is fired to burn out the wax pattern therein, and a molten metal is cast into the resulting void to prepare a metal casting, which is then washed, adjusted and polished. The working time required per tooth is about 30 minutes for the preparation of the wax pattern, about 1 hour for the investment thereof, about 1 hour for the burning-out thereof, about 10 minutes for casting, and about 30 minutes for the polishing of the casting. The time required for such seccessive works from the preparation of the wax pattern to the polishing of the casting thus totals up to about 3 hours. Noble and base metal alloys are generally used for such metal castings. By way of example, for fusing porcelain, on the one hand, Au-Pt-Pd and Au-Pd-Ag alloys and Ni-Cr, Ni-Cr-Mo and Ni-CrCo-Mo alloys are used as the noble and base metal alloys, respectively, and for building up plastics, on the other hand, Au-AgCu, Au-Ag-Pd and Ag-Pd alloys and Ni-Cr and Co-Cr alloys are employed as the noble and base metal alloys, respectively.
Turning to the making of the inner crown of the compositelayered crown with the use of the metal foil, a noble metal foil, which is in the form of, e.g., an umbrella with a bull's-eye design, has a thickness of 50 micrometers and is of a four-layer structure comprising 100 % Au; 85 % Au+5 % Pt+10 % Pd; 100 % Pt; and 80 % Au +10 % Pd+10 % the balance, is covered over the working model of a tooth, and is formed thereon with 8 to 10 folds with the use of a pincette. The foil is repeatedly pressed against the working model of the tooth along its surface by means of a spatula for pressing a foil. The noble metal foil is then swaged over the working model of the tooth by a swager which comprises an outer pipe and a cylindrical member to be finally fitted thereinto, has on its bottom the working model of the tooth with the noble metal foil being pressed thereagainst, and exerts a blowing and swaging action via rubber. Thereafter, the noble metal foil is taken out of the working model of the tooth, and heated with a gas burner to melt the gold in the foil.
However, the making of the inner crowns of the compositelayered crowns using the metal casting involves such problems as enumerated below.
(1) In some cases, since the dimensional accuracy of the metal casting is not so good, it may not be well-fitted over a working model of a tooth on which the state of a tooth to be restored is reproduced. This is because although the dimensional accuracy of the metal casting has to be corrected by the expansion of an investment for the reason that it is affected by dimensional changes occurring in a wax pattern, an investment and an alloy, in particular the casting shrinkage of an alloy, it is difficult to completely correct the dimensional accuracy of the metal casting due to the fact that such casting shrinkage is largely affected depending upon not only the type of alloy but also the shape of the metal casting, the strength of the investment and the casting conditions.
(2) Since the thickness of the metal casting is on the order of 200 to 300 micrometers, it is required to increase the amount of procelain or plastics to be built up so as to obtain the configuration of a crown similar to that of a natural tooth and, at the same time, obtain the color tone corresponding to that of a natural tooth. For that reason, it is required to increase the amount of a patient's tooth to be removed in the tooth preparation for fixed prosthodontics. This gives rise to an increase in the time required for the tooth preparation for fixed prothodontics to be carried out by a dentist, and causes pain to a patient. For this it is ideal to minimize the thickness of the metal casting. Actually, however, difficulty is encountered in reducing the thickness of the metal casting to 200 micrometers or less due to the occurrence of casting defects such as rounded, cold shut, rough surface and micro shrink.
(3) As already mentioned, the works from preparation, investing and burning-out of the wax pattern to the polishing of the casting takes a period of time of at least 3 hours, and the preparation of the metal casting is time-consuming to a person who carries it out.
On the other hand, the making of the inner crowns of the composite-layered crowns using a metal foil involves such problems as enumerated below.
(1) Even when a noble metal foil is covered over the working model of the tooth, is provided with 8 to 10 folds by an exclusive pincette, and is thereafter repeatedly pressed against the working model of the tooth along its surface with the use of an exclusive spatula for pressing a foil, the resulting pressed metal foil, viz., the resulting metal coping shows only incomplete fitness with respect to the working model of the tooth. For that reason, there is a certain limit in the fitness of the pressed metal foil or the metal coping with respect to the working model of the tooth, even though the pressed metal foil or the metal coping pressed against the working model of the tooth is swaged by a swager.
(2) When the pressed metal foil or the metal coping pressed against the working model of the tooth is swaged by a swager, the working model of the tooth may break down in dependence on the type, shape and size of the working model of the tooth or how to use the swager. Hence, fair experience and expertness are needed for precise swaging.
(3) When the pressed metal foil or the metal coping is removed from the working model of the tooth after it has been swaged by a swager, it tends to deform due to its limited strength. To facilitate such removal, there is a need for simplifying the preparation of the working model of the tooth, to which a dentist should pay attention in the tooth preparation for fixed prosthodontics.
(4) After the pressed metal foil or the metal coping has been removed from the working model of the tooth, it is finished up by heating with a gas burner. Although attention must then be paid to the position and time at and during which it is exposed to flames of the gas burner, fair experience and expertness are needed to this end. This is because the gas burner heating is a difficult work such that a gold layer in the noble metal foil layer is molten and case in between 8-10 folds formed on the pressed metal foil or the metal coping to complete it.